Pelvic inflammatory disease (PID) is defined as an infection of the endometrium, fallopian tubes, and adjacent structures that is caused by the ascension of microorganisms from the lower to upper genital tract The microorganism most convincingly associated with PID is Chlamydia trachomatis, and our proposal would provide the exciting opportinity for a discovery of the antigen-specific cell mediated immune resposes that are most strongly associated with protection against C. trachomatis infection as well as the elucidation of the antigen specifc immune responses correlated with containment of C. trachomatis infection to the lower genital tract. The 200 women enrolled into this longitudinal investigation will be recruited from an acute PID cohort (Project 1) and from a cohort of women who are at high risk for endocervical infection with C. trachomatis (Project 4). Completion of our investigation will allow: 1. Better understanding of the CD4 T cell antigen-specific responses that are most strongly associated with protection from C. trachomatis infection at enrollment. 2. Better elucidation of the CD4 T cell antigen-specific responses that are most strongly correlated with protection from incident C. trachomatis infection. 3. Identification of novel chlamydia-specific T cell responses that are most strongly associated with the decreased likelihood of ascension of C. trachomatis from the lower to upper genital tract.